85mm f/1.4 vs 135mm f/1.8 for portraits: depth of field and background blur
Asked 4/11/2026
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On a full-frame camera, if I focus at about 3m with an 85mm f/1.4, and then switch to a 135mm f/1.8 and move back to about 4.7m to keep the same framing, a depth-of-field calculator shows roughly 10cm DoF for the 85mm and about 13cm for the 135mm. Is that reasonable?
More importantly, if the 85mm appears to give slightly shallower DoF at the same framing, why would someone choose a 135mm f/1.8 for portraits when the goal is stronger background blur?
Originally by Roman T. Source · Licensed CC BY-SA 4.0
Roman T
2mo ago
2 Answers
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The background blur and the depth of field do not vary in the same way with different lenses.
Size of Background Blur
For a distant background the size of the background blur measured on the same scale as the subject of the picture (i.e. whatever the lens is focussed on) is equal to the size of the entrance pupil.
For a 135mm f/1.8 lens the entrance pupil is 135/1.8 = 75 mm.
For an 85mm f/1.4 lens the entrance pupil is 85/1.4 = 61 mm.
So the former lens will give about 23% larger background blur (for a distant background).
If the framing is kept the same, the depth of field varies with f-number of the lens and not with its focal length as well, but the size of the blur varies with the entrance pupil (which is the focal length divided by the f-number). So the 85mm f/1.4 lens will give less DoF than the 135mm f/1.8 lens because 1.4 is less than 1.8 and the background blur will be less because 85/1.4 = 61mm is less than 135/1.8 = 75mm.
See also:
Is there an easy way to calculate the amount of background (or foreground) blur to expect?
Originally by Tom Axford. Source · Licensed CC BY-SA 4.0
Tom Axford
2mo ago
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Generated from our catalog & community — verify before relying on it.
Yes—those numbers can be reasonable. For the same framing on the same format, depth of field depends mainly on f-number, so an 85mm f/1.4 can show slightly shallower DoF than a 135mm f/1.8.
But DoF and background blur are not the same thing. DoF describes what appears acceptably sharp; it does not directly measure how large or creamy out-of-focus background details look.
For stronger background blur at the same framing, the longer lens can still win because blur size is tied more closely to entrance pupil diameter (focal length ÷ f-number):
- 85mm f/1.4 → about 61mm
- 135mm f/1.8 → about 75mm
So the 135mm f/1.8 can produce larger blur discs for distant backgrounds, even if the calculated DoF is slightly deeper. Also, when you move back to keep the subject the same size, you cancel the extra magnification at the subject plane, but not fully for the background, so the background can still look more blurred/compressed with the longer lens.
In short: 85mm f/1.4 may give slightly shallower DoF; 135mm f/1.8 can still give stronger background separation.
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